Electronic function generators are devices which generate various wave forms. An essential element in many function generators is a circuit which produces a triangular wave form, typically by charging a capacitor between fixed limits. The triangular wave shape is produced by means of a voltage detector which reverses the direction of the charging source when appropriate limits are reached. In a typical function generator sinusoidal wave forms are produced from the triangular wave form by applying the triangular signal to a nonlinear network, typically a diode network.
It is characteristic of the above-described function generators that the nonlinear diode network produces harmonic distortion of a sinusoidal signal at a desired fundamental frequency. By this is meant that undesired harmonics of the fundamental frequency appear in the output of the device. Generally it is desirable to reduce the amplitude of these harmonics to minimum levels. The magnitude of the harmonic distortion products depends principally on two factors; one is the intrinsic characteristics of the nonlinear network which shapes triangular wave forms into a sinusoidal wave. Another critical factor is the integrity of the triangular wave form itself.
The present invention is concerned with generating a triangular wave form which is extremely stable and insensitive to drift with temperature or time. The invention provides amplitude symmetry about the average value so that long term drifts of the average value (i.e., base line drifts) are minimized. The invention further provides long term time symmetry of the wave form; i.e., it assures that the absolute value of the positive and negative slopes always tend toward equality.
In accordance with the preferred embodiment, a loop is employed in which a level detector provides a square wave signal to a triangle generator. Amplitude symmetry of the triangular wave is provided by detecting and integrating the triangular wave form to produce an average value. The average value is compared to a stable reference whence an error signal is derived which is applied to a level detector to control the amplitude of the triangular wave form. In response to the error signal the level detector tends to increase or decrease the amplitude so as to cancel the error and restore amplitude symmetry.
The time asymmetry of the triangular wave is the same as that of the square wave in the loop. To provide time symmetry, the square wave is used to switch a stable current source whose integrated value is therefore proportional to the time symmetry (duty cycle) of the square wave. An error signal generated from the integrated current is used to control either the positive or negative slope produced by the triangle generator in order to always restore time symmetry to the triangular wave form.